When air flows through a manifold of an engine, the flow along the branches of the manifold results not only in a mass transfer along the length of the branch but also in the transmission of sound waves or pressure waves along the branch. By appropriate tuning of the length of the branches, the sound waves can be used to enhance the charge density at the instant that the intake valves close and to improve scavenging of the exhaust gases at the instant that the exhaust valves close, thereby improving engine output power. In other cases, tuning may be required to reduce charge density at the time the intake valves close, for example to reduce pumping losses during part load operation or to increase the pressure at the time that the exhaust valve closes, so that internal exhaust gas recirculation is increased. In other words, the tuning of the manifold can be designed to reflect a positive or a negative pressure wave to the engine port at the time that the port valve closes, to enhance engine performance in a variety of ways.
Though their advantages have been known, tuned manifolds have hitherto been bulky and difficult to incorporate within the limited amount of the space available in the engine compartment and the present invention seeks to introduce greater flexibility into the design of a tuned engine manifold and to enable a tuned manifold of smaller dimensions to be produced.